Conventional flexible metal-clad laminates for flexible printed wiring boards comprise a polyimide film and a metal foil bonded by an epoxy resin, acrylic resin or like thermosetting adhesives. Flexible printed wiring boards formed from a flexible metal-clad laminate bonded by such a thermosetting adhesive have the problems that they have a substrate (resin film layer) having an increased thermal expansion coefficient due to the adhesive and therefore have inferior thermal dimensional stability, and that they are inferior in moisture resistance and therefore have impaired insulation properties and solder thermal resistance after humidifying treatment. Moreover, there are other problems that, due to the inferior thermal dimensional stability, the flexible metal-clad laminates or flexible printed wiring boards prepared by forming a circuit using the flexible metal-clad laminate are curled during various heating processes, and due to the inferior moisture resistance, the printed wiring board curls or is distorted during etching and like wet processing, resulting in impaired yields during circuit formation and/or during mounting on the resulting flexible printed wiring boards.
To solve such problems, techniques to form a metal foil directly on an insulating substrate without using an adhesive (i.e., two-layer flexible metal-clad laminates) have been developed. For example, Japanese Unexamined Patent Publication No. 1990-98994 discloses sputtering, Japanese Unexamined Patent Publication No. 1987-181488 discloses vacuum evaporation and Japanese Unexamined Patent Publication No. 1982-18357 discloses ion plating, for the formation of a metal layer (seed layer) over a polyimide film and the subsequent formation of conductive layers is performed by plating. However, all of these methods have problems, such as high production costs, pinhole creation during seed layer formation, and insufficient adhesion between the polyimide film and the conductor.
In an attempt to inexpensively provide high performance flexible printed wiring boards that are not furnished with an adhesive layer, publications such as Japanese Unexamined Patent Publication No. 1982-50670 propose a method for forming a flexible metal-clad laminate wherein a solution of a polyamic acid, i.e., a precursor of a polyimide resin, is directly applied to a metal foil and subjected to dehydration/polyimidization reaction on the metal foil. However, flexible metal-clad laminates obtained by such a method are problematic because their insulation properties after humidification are poor due to the high moisture absorption of the resin, resulting in poor reliability (migration resistance) in applications where high voltage is applied (such as flexible printed wiring boards used at display edges. Furthermore, there are other problems that the solder heat resistance is also decreased after humidification and therefore the application to lead-free soldering (Ag—Sn—Bi-based, Ag—Sn—Cu-based, etc.) is limited, and the printed wiring board curls or is distorted, especially during wet processing or in a highly humid atmosphere, resulting in impaired yields in circuit formation and mounting on flexible printed wiring boards. In addition, after laminate formation (after coating) with the use of the precursor polyamic acid, a heat treatment has to be carried out at high temperatures. Therefore, the use of such a resin for the continuous production of two-layer flexible metal-clad laminates has the problem that productivity is reduced, expensive equipment is necessary, and production costs are increased.
To solve such problems, Japanese Unexamined Patent Publication No. 1993-59174 discloses a resin having excellent heat resistance and small thermal expansion, in which polyamide-imide structures having a trimellitic acid skeleton are randomly copolymerized with polyimide structures, as well as the use of said resin as a base material or a covering material for flexible printed circuit boards. However, there is a problem that once a varnish in which this polyamide-imide resin is dissolved is stored, the viscosity of the solution gradually increases, and in about a month, the solution solidifies and looked like a pudding.
As described above, prior art techniques have not developed flexible metal-clad laminates for producing two-layer-type flexible printed wiring boards, with excellent polyamide-imide resin varnish stability, the flexible printed wiring boards being excellent in dimensional stability and heat resistance, devoid of decrease in solder heat resistance or insulation properties even under humidifying conditions, and free of warpage or distortion in any environment (under humidifying or heating conditions).
An object of the present invention is to solve the problems described above. In particular, an object of the invention is to inexpensively provide a flexible metal-clad laminate for flexible printed wiring boards that are usable even at display edges for which high reliability is required. Specifically, an object of the invention is to inexpensively provide, by directly applying a heat resistant resin solution to a metal foil, a flexible metal-clad laminate for flexible printed wiring boards that have excellent heat resistance, dimensional stability, adhesion, etc., allowing no loss of insulation properties and solder heat resistance and does not curl or dimensionally change, even under humidifying conditions.